It's interesting to see the mixed views people have on this subject. I thought I'd add my two cents because it didn't seem like an option in the poll.
Lets face it, sex is important in any relationship, whether you're having it or not. It is almost the very foundation of human relationships. As someone who was in a nonsexual relationship, and an actively sexual relationship, I can give a little perspective (from my point of view anyway) on both sides.
In my first relationship, the girl was, how can I put this... less than knowledgeable in the physical department. Of course, I didn't mind this, as I loved the girl and sex was (and still isn't) that big of a deal to me. However, the relationship wasn't working out as we would fight all the time and be stressed out. Since we couldn't relieve any stress or sexual tension, we would bottle up our emotions and just basically explode at the slightest emotionally jarring thing to happen. It became so frustrating that, among a host of other things, it didn't work and I ended it.
My next (and current) relationship is a different story. At first, we weren't physical as much either. But as time went on, and we began "exploring each others' bodies", not only were we having fun (a lot of fun

) we were becoming closer to each other as a couple. We both became more confident in ourselves and made us "happier" and more outgoing than before.
I'm not saying sex is the be-all-end-all in a relationship, and it shouldn't be done until both parties agree to it. What I am saying is sex can change a relationship, having it or not having it.
I guess to sum it all up, I find it important but not that important. Necessary, but only when ready.